The OWL 2 Web Ontology Language, informally OWL 2, is an ontology language for the Semantic Web with formally defined meaning. OWL 2 ontologies provide classes, properties, individuals, and data values and are stored as Semantic Web documents. OWL 2 ontologies can be used along with information written in RDF, and OWL 2 ontologies themselves are primarily exchanged as RDF documents. The OWL 2 Document Overview describes the overall state of OWL 2, and should be read before other OWL 2 documents.

2 Ontologies

An OWL ontology consists of a set of axioms. It may include annotations and may import other ontologies. An ontology document may also include namespace declarations, including those listed above that contain the reserved vocabulary for the OWL 2 language.

2.1 Importing Ontologies

An ontology can make use of definitions from other ontologies by importing the ontologies that contain them. Importing an ontology means that the user commits to all of the statements made in that ontology.

2.2 Ontology Versioning & Related Annotations

Ontology-specific annotations may include version information or annotations indicating something about compatibility with other versions of the same ontology. For example, an ontology identified by an ontologyURI may also have a versionURI.

a:property is a DataPropertya:individual has value "constant" for a:propertya:property1 is a subproperty of a:property2

Built-in data properties with predefined semantics in OWL include:

The data property with URI owl:TopDataProperty - connects all possible individuals with all possible data values, or the top role

The data property with URI owl:BottomDataProperty - does not connect any individuals with any data values, or the bottom role

3.3.3 Annotation Properties

Annotation properties provide a means for augmenting ontologies, entities, or axioms with additional metadata. A number of built-in annotation properties from the RDF Schema vocabulary can be used in OWL. These include:

rdfs:seeAlso - to associate information available at some URI, or another entity, with an entity to provide additional information

rdfs:isDefinedBy - augment the definition of an entity with information available at some URI, or through another entity

In addition, the built-in owl:deprecated property may be used to indicate that a particular entity is deprecated within some context (i.e., set to "true"^^xsd:boolean). See Section 2.2, above, for more on ontology-specific built-in annotations.

The general form for annotation using the built-in vocabulary is as follows:

3.4 Individuals

Individuals represent the actual objects in a domain, and may be named or anonymous. Named individuals by definition have URIs associated with them and are therefore considered entities. Examples using named individuals include: